Since polyvinyl alcohol (hereinafter abbreviated as PVA) resins are excellent in gas barrier properties, toughness, transparency, and the like, they are suitable as packaging materials for various articles.
However, since characteristics such as gas barrier properties remarkably change through water absorption and moisture absorption, PVA resins are usually used as one layer of a multilayer structure sandwiched between resin layers excellent in moisture resistance at the use as packaging materials.
In general, as a method for preparing the multilayer structure, a co-extrusion method is suitable in view of production cost and the like but, since melting point is close to decomposition temperature in usual PVA resins, melt molding is substantially impossible. Accordingly, for the production of the multilayer structure having a PVA resin layer, it is unavoidable to adopt a method of converting a PVA resin into an aqueous solution, subsequently casting and drying it to form a film, and then laminating the film with other film(s) or a method of applying the solution on the other film and drying it to form a layer. This limitation has been a large obstacle to wide spread of the PVA resins to packaging material uses.
Ethylene-vinyl alcohol copolymer (EVOH) in which ethylene is introduced into the main chain of PVA resin is melt-moldable and hence has been widely used for various packaging materials. However, since EVOH has slightly poor gas barrier properties as compared with PVA, EVOH is insufficient for uses where higher gas barrier properties are required. Therefore, it is greatly desired to provide a PVA resin excellent in gas barrier properties and melt-moldable.
For such a problem, recently, as a PVA resin melt-moldable and excellent in gas barrier properties, there has been proposed a PVA resin having a 1,2-diol component in the side chain. (See, for example, Patent document 1)
The excellent gas barrier properties of the PVA resins result from high crystallinity, which is also a main cause of the high melting point of the PVA resins. In the PVA resin described in Patent Document 1, the steric hindrance at the side chain lowers the melting point but, on the other hand, the molecular chains are strongly confined by strong hydrogen bonds of the hydroxyl groups in the side-chain 1,2-diol component. Thus, it is assumed that this confinement suppresses degradation of gas barrier properties resulting from the lowered crystallinity.
However, the high crystallinity of the molecular chains and the strong confinement by the hydrogen bonds in the PVA resins cause poor flexibility and impact resistance of the PVA resin as compared with other thermoplastic resins. Although the PVA resin having the 1,2-diol component in the side chain described in Patent document 1 has been also slightly improved by the steric hindrance at the side chain, the resin is still insufficient for practical use.
On the other hand, for the problem of enhancing the flexibility and impact resistance of various resins, there has been widely studied a method of improving the characteristics, without impairing the characteristics of the base resins, by blending a substance having a low elastic modulus and non-compatibility therewith and thus forming a sea-island structure in which the substance forms an island component.
As for PVA resins, there has been proposed a resin composition wherein blending of a styrene-based thermoplastic elastomer therewith results in formation of a sea-island structure in which the PVA resin is a sea component and the styrene-based thermoplastic elastomer is an island component and thus the flexibility and impact resistance are improved. (See, for example, Patent document 2.)
Accordingly, when the improvement of the flexibility and impact resistance has been studied through blending of a styrene-based thermoplastic elastomer with the PVA resin having the 1,2-diol component in the side chain described in Patent Document 1, it has been found that a considerable effect is observed but the blending is still insufficient for satisfying a more advanced requirement. For example, there is a case that when a film obtained from the resin composition is treated under such a severe condition that the film is repeatedly bent, a large number of pinholes are formed and thereby the gas barrier properties are degraded for a moment.
In the case of such a sea-island structure-type polymer alloy, the dispersion state and the interface state of the island component have a large influence on characteristics and, also in the aforementioned case, there are considered such causes that the styrene-based thermoplastic elastomer is not sufficiently micro-dispersed in the PVA resin or the phases are apt to exfoliate each other owing to insufficient affinity between the sea component and the island component and hence insufficient energy transfer at the interface results in insufficient impact absorbability.
For improving the problems, it has been known that combined use of a compatibility accelerator is effective. Since it is necessary for such a compatibility accelerator to have good affinity to both of the sea component and the island component, there has been generally employed one having structural parts that are common to respective components.
For example, in a polymer alloy in which the sea component is a polyamide resin and the island component is a styrene-based thermoplastic elastomer, there has been proposed a resin composition using a styrene-based thermoplastic elastomer to which a polyamide is grafted as a compatibility accelerator. (See, for example, Patent document 3.)
In such Patent Document 3, since the polyamide graft block copolymer used as a compatibility accelerator is common to the island component at the main chain part and is common to the sea component at the graft part, the copolymer has excellent affinity to any of both components. As a result, it is considered that the island component is stably micro-dispersed through localized presence thereof at the interface of the sea-island structure and furthermore impact resistance is improved through good energy transfer at the interface.